In the first part of the story the narrator emphasizes the cold temperature. The man is continually being surprised that it is so much colder than he expected. This seems ominous.
He was surprised, however, at the cold. It certainly was cold, he concluded, as he rubbed his numbed nose and cheekbones with his mittened hand.
Eventually the narrator states explicitly what the exact temperature is, while at the same time he describes how the dog knows instinctively that it is too cold to be out hiking.
The animal was depressed by the tremendous cold. It knew that it was no time for traveling. Its instinct told it a truer tale than was told to the man by the man's judgment. In reality, it was not merely colder than fifty below zero; it was colder than sixty below, than seventy below. It was seventy-five below zero.
Seventy-five below zero Fahrenheit sounds dreadful to the reader. How could anybody stand such an extreme? The reader sides with the dog. It was no time for traveling. The narrator also specifies that the man is a Chechaquo, a novice, a newcomer. This too seems ominous. If the man were not a Chechaquo he would not be traveling in this kind of weather. All the experienced men are inside their cabins with their fires blazing.
Subsequently the narrator introduces what will turn out to be the main problem. There is an extreme danger of breaking through a patch of snow-covered ice and getting wet. We have seen that water will freeze instantaneously in this kind of cold weather. The title of the story, "To Build a Fire," strongly suggests that the man is going to get wet and that his life will depend upon his being able to build a fire. Where can he find dry wood in that environment?
The creek he knew was frozen clear to the bottom—no creek could contain water in that arctic winter—but he knew also that there were springs that bubbled out from the hillsides and ran along under the snow and on top the ice of the creek. He knew that the coldest snaps never froze these springs, and he knew likewise their danger. They were traps. They hid pools of water under the snow that might be three inches deep, or three feet.
This covers most of the foreshadowing. The narrator constantly uses the dog to inform the reader that the man is making terrible mistakes. The foreshadowing comes to a sudden end when the thing the man fears actually occurs. His fears of this occurrence are all foreshadowing of the actual event.
And then it happened. At a place where there were no signs, where the soft, unbroken snow seemed to advertise solidity beneath, the man broke through. It was not deep. He wetted himself half-way to the knees before he floundered out to the firm crust.
The reader is strongly identified with the man because he can empathize with the man's motivation to get to the camp on the left fork of Henderson Creek, and also because the reader is held in the man's point of view, except for a few shifts to the dog's point of view. The reader has experienced the extreme cold in his imagination as well as the man's loneliness; and now the reader will experience the man's terror and desperation, and even finally experience the man's death. The foreshadowing has been extremely effective right up to the point where the story reads:
And then it happened.